System and method of generating seismic waves in the earth



NS ETA OF GENER N THE EAR L 2,730,187 ATING E HAWK] AND METHOD MIC WAVESI Jan. 10, 1956 J.. SYSTEM SEIS 1947 2 Sheets-Sheet 1 Original FiledJan. 11

RECORDER l (L FIG. 1'

I'S wkins ugh Invenfo Jame W II a B IL Attorney;

Jall- 1956 E. HAWKINS ET AL 2,730,187

RATING J. SYSTEM AND METHO F GENE MIC SEIS WAVES THE EARTH OriginalFiled Jan. 11, 1947 2 Sheets-Sheet 2 Series Connecled To IgnitionElements Of Other Charges l7a N0 Delay 76L I70. 37 Ouler Charges Delayed7 Ouler Charges 38 INVENTOR- Delayed 3% James E Haw/ans 5% BY W/l/lam EPu h zye )g A ffomevs SYSTEM AND IVIETHOD F GENERATING SEISMIC WAVES INTHE EARTH Original application January 11, 1947, Serial No. 721,558,

now, Patent No. 2,630,188, dated March 3, 1953. Di-- vided and thisapplication June 13, 1952, Serial No. 293,379

7 Claims. (c1. 181-.5)

The present invention relates to the art of seismic prospecting and moreparticularly to an improved system and method of generating seismicwaves in the earth. .This application is a division of applicantscopending application Serial No. 721,558, filed January 11, 1947, nowPatent No. 2,630,188, which is assigned to the same assignee as thepresent application. 1

Fundamentally, the art of seismic surveying is based upon the generationof sound or seismic waves in the earths crust and detecting, recordingand interpreting the waves which are reflected and refracted back to theearths surface from buried strata interfaces and the like. Presentpractice in the generation of seismic waves is that of detonatingdynamite or other explosives, as commercially obtained in'packa'ge form,or as placed in a suitable container, metal or otherwise, to facilitateloading, in a shot hole usually drilled below the weathering or lowvelocity layer of the earths crust, Detonation of the explosive chargebelow the weathering layer is usually necessary in order to avoidgenerating excessive amounts of undesired surface or near surfaceseismic wave energy which travels horizontally or nearly so to thepoints of wave detection. These undesired waves frequently havefrequencies approximating the frequencies of the desired reflected andrefracted waves and hence defy filtering prior to recording. They havethe effect of so confusing or obscuring the records'of the desiredreflected waves as to render record interpretation difiicult orimpossible. In many areas, the interfering wave energy is of such largemagnitude as compared with the magnitude of the reflected wave energythat successful reflection prospecting is either very difficult andexpensive or impossible even when shot detonation below the weatheringlayer is resorted to to reduce the undesired wave energy. Moreover, eventhough this method of shooting is oftensuccessful in reducing theinterfering waveenergy to a tolerable value, it is somewhat expensive inthat the shot holes must be drilled to considerable depths. In extremelyunfavorable areas, undesirable energy cannot be eliminated even when theshot holes are drilled below the weathering and further, heavy andexpensive explosive charges must be used in order to obtain reflectedwave energy in sufficient magnitude to permit accurate interpretation ofthe resulting record.

It is an object of the present invention, therefore, to provide animproved system and method 'for so generating seismic waves in the earththat more effective utilization of explosives is obtained, thereby toconserve explosives and to eliminate the necessity of drilling shotholes through the weathering layer except in unusually diflicult areas.i

It is another object of the invention to provide a method and system ofthe character described in which the explosive force is directionalizedin a downward direction, thereby to decrease the amount of undesiredhorizontal wave energy produced and to increase the amount of nitedStates Patent 0 M wave energy propagated downwardly toward thereflecting interfaces. 7

It is still another object of the invention to at least partiallysuppress certain undesired interfering (horizontal) waves of a selectedand particularly objectionable frequency.

The invention, both as to its organization and method of operation,together with other objects and advantages thereof will best beunderstood by reference to the following specification taken inconnection with the accom{ panying drawings in which: 1

Fig. 1 illustratesa seismic surveying system embodying a system ofgenerating seismic waves characterized by the features of the presentinvention;

Fig. 2 illustrates the shot hole pattern of the seismic wave generatingsystem embodied in the seismic surveying system shown in Fig. 1;

Fig. 3 is a view partially in section illustrating the details of one ofthe cavitated charges preferably used in each of the shot holes shown inFigs. 2 and 4;

Fig. 4 is a view similar to .Fig. 1, but illustrating electricalequipment for detonation of the center charge and the outer charges atthe same or different times; and

Figs. 5 and 6 graphically illustrate the degrees of directionalizationobtained when the center and outer charges are at the same and diiferentlevels in the earths crust and are fired simultaneously and with delaytimes between the center and outer charges.

Referring now to the drawings and more particularly to Fig. 1 thereof,the present improved seismic signal generating system is thereillustrated in its use in a seismic surveying system which comprises theusual amplifying and filtering apparatus 12 for transmitting detectedsignals from an array of geophones or signal detectors 11 in amplifiedform to the recording elements of a seismic signal recorder 13. Thedetectors 11 may be arranged in any desired array relative to anddistant from the point 10 of seismic wave generation. As pointed outmore fully below, detonation of the explosive charge or charges employedin the present improved wave generating system is effected under thecontrol of a conventional electric impulse type detonator 14 which iselectrically connected by means of a cable 14a with one of the signaltranslating channels of the apparatus 12 for the purpose of transmittingthe time-break signal to one of the recording elements of the recorder13. This detonator together with the signal detectors 11,,the amplifyingand filtering apparatus 12 and the recorder 13 may be of any desiredcommercial construction.

In general, the mode of operation of the system as thus far describedwill be readily understood by those skilled in the art. Briefly,however, seismic signals generated in the manner more fully pointed outbelow at the wave source or shot point 10 distant from the array ofdetectors 11, and as transmitted through the earth both directly andthrough reflection and refraction from buried strata interfaces and thelike, are picked up by the detectors 11, converted into correspondingelectrical signals through the operation of these detectors, amplifiedthrough the respective amplifying channels of the apparatus 12 andimpressed upon the respective associated driving coils of thegalvanometer elements embodied in the recorder 13 for recording. Also inaccordance with conventional practice the time-break signal, as producedsimultaneously with generation of the seismic waves, is transmitted overthe cable 14aand through at leasta part of one of the channels of theapparatus 12 to one of the recording elements of the recorder 13 forrecording.

As pointed out above, the conventional method of geneating' seismicwaves required in practicing the above described'method of seismicsurveying is that of drilling a shot hole through the weathering or lowvelocity Patented Jan. 10, 1956 was-rev,

layer 9 of the earth's crust; depositing an explosive charge usually inthe form of packaged dynamite at the bottom of the shot hole anddetonating the charge. This system of seismic wave generation is"open'to the several 'jdisadvantages briefly outlinedabove. Inaccordancewith the present invention thesedis'advanta'gesare at least inpart obviated through useof the present improved seisrriic' signalgenerating system to directionalize the seismic wave propagation and toat least in part suppress interferingwaVe' energy. In one embodiment ofmemvention, these ends are accomplished by depositing a holes 15 and 16are not drilled through the weathering plurality of explosive charges 17in a plurality of shot 7 holes 15 and 16 to form an explosivecharge'pattern of approximately conical configuration which simulates alarge cavitated charge and has theeffect of producing massdirectionalization of'the jge'nerated'sei'smic waves in the mannerexplained below with reference toFigs. 5 and 6 of the drawings when theexplosive charges 17 are detonated. More specifically and as bestfshownin Fig. 2 of the drawings, the shot hole is drilled at thecenter of aclosed loop, 'preferablyinthe formof acircle 16a, and the shotholes 16are drilled at'equally spaced points around the circumference of thiscircle. In order to obtain the desired directionalization of theexplosive energy, the center shot hole 15 is drilled to a depth A notgreater than the depth B to which the surrounding shot holes 16 aredrilled and usually less. 'For example, these depths are preferably soproportioned that the cone angle Eis of the order of 90 if all of thecharges 17 are to be simultaneously detonated. The detonating caps ofthe respective explosive charges 17 are commonly connected by means oftwoconductor cables 14b to the current impulse generating circuit of thedetonator 14 (which may be of the schematic form shown in Fig. 4 of thedrawings) so that when this detonator is operated, the charges areexploded either simultaneously or in sequence.

, As explained more fully below with reference to Figs. 5 and,6 of thedrawings, with the explosive charges 17 arranged in a conical pattern tosimulate a large cavitated charge, the well known Munroe Effect isobtained when these charges are either simultaneously detonated ordetonated in a predetermined order. Specifically, the mass concavity ofthe shaped arrangement of explosive charges in the several shot holes inpart causes the seismic energy to be directionalized in bulk'in thesensethatthe wave energies produced at the individual shot holes arefocused toward the central portion of the group pattern. Inthearrangement illustrated, wherein the apex of the cone is nearest theearths surface 8, the mass directionalization of the generated seismicenergy is in a downward direction. Due to the describeddirectionalization of the generated waves, the available explosiveenergy per unit of explosive material is much more etfe'ctivelyutilizedfor the reason that the increase .in seismic energy propa- 'gationdownward-is realized at the-expense ofseismic energy propagation alonghorizontalornear-horizontal paths. As explained above, however,reduction in the horizontal seismic energy propagation is highlydesirable since it is this energy which gives riseto interfering wavesat the detectors 11 having the eifect' of obscuring or completelyobliterating the reflecting and refracted waves which must be recordedto permit accurateinterpretation of the attitude of the subsurfacestructure. Furthen'the increased effectiveness 'of each available unitof explosive material in producing wave Qpropagation in a downwarddirection has the effect of further reducing the amount of explosiverequired to produce the desired amount of reflected and refractedenergy. Thus while several charges are employed, the aggregate ofthee);- plosive material in the several charges maybe appreciably lessthan that in a single charge of sufficient size to produce the sameamount of seismic energy propagation in a downward direction. Thisfurther contributes to the desired reduction in undesired wavepropagation in layer 9. This is possible by virtue of the increasedeffectiveness of the explosive charges in producing seismic energypropagation in a downward direction and reducing the seismic energypropagation in the horizontal directions. In most areas where favorableconditions prevail, shot holes 15 and 16 drilled to very shallow depthsin the weathering layer 9 should sufiice to produce entirely acceptablerecords. However, in certain very difficult areas of deep weathering(exceeding one hundred feet, for example) and in areas where there is arelatively large amount of interference energy, it maybenecessaryto'drill the holes 15 and 16 to depths C and D, respectively,extending below the bottom 9a of the layer 9. In such'case', also,aditferentialbetween the depth C of the center hole 15 and the depth Dof the surrounding holes 16 may be desired in order to realize thedescribed mass concavity'e'ffect whichresults in directionalizedpropagation of the seismic energy-in a downward direction when thecharges in the several shot holes are detonated.

As indicated above, with the described system a reduction in theinterfering or horizontally propagated energy is inherently obtained dueto the directionalization of the generated seismic energy in a downwarddirection. In the usual case, the desired reflected and refracted wavesfall within a restricted frequency range. This frequency range may varyfrom area to area depending upon the character'of the surface andsubsurface, but usually fall! in a range of from 30 to 100 cycles persecond. It is desirable, therefore, particularly in unfavorable areaswhere difficulty is experienced in obtaining appreciable wave reflectionand refraction, to suppress or minimize the interfering horizontallypropagated waves of the same frequency since, due to the frequencysimilarity of the desired and undesired energy, filtering within theapparatus 12 is not effective to separate the' desired energy from theundesired energy. v

In accordance with another feature of the present invention, suppressionof undesired waves is obtained by so arranging and detonating thecharges that the interfering waves produced by detonation of the outercharges are cancelled by waves of the same frequency produced bydetonation of the center charge. If the charges are to be detonatedsimultaneously, this may be accomplished by making the radius ofthe'circle 16 an odd multiple or submultiple of the wave length of theparticular frequency at which the desired energy is to-be received. Forexample, if test shots indicate thatthe-interfering energy occurs at afrequency oftwenty cycles persecond andalso that the velocityofwavepropagat ion through the weathering or low velocity layer 9 is twothousand feet per second, the wave length of the twenty cycle energyobviously is one hundred feet. In such a'case, by'arranging thesurrounding shot holes 16'symmetricallyabout the center of the shot hole15 on a circle having a radius of fifty feet, the twenty cycle waveenergy generated in the center shot hole 15 will be'approxim'ate1y'180out of phase with the twenty cycle wave energy generated inthesurrounding holes 16 n in a horizontalplane, resulting incancellation ofa considerable amount of twenty'cycle wave energy in directionsextending laterally from the shot holes. As a further example, if theshot holes are drilled below the weathering or low velocity'layer 9 inthe manner illustrated in'dash lines in Fig. 1 of the drawings and testsindicated that suppressionof an interfering fiftycyc'le wave energy isdesirable-and I that :the velocity -0f1W5V6 propagation the strata:below :the weathering layer 9*is' eight thousand feet per second, wavesof the frequency indicated will have a wave length of one hundred andsixty feet. Therefore, to

obtain suppression of the interfering energy of this wave length in ahorizontal directisn, the outer shot holes 16 should be symmetricallyspaced about the inner shot hole on the circumference .of a circlehaving a radius of eighty feet. Thus by a proper choice of the length ofradius of the circle on which the outer shot holes 16 are located,propagation of undesired interfering wave energy may be furtherminimized.

As will be evident from the above explanation, partial suppression ofundesired horizontal wave energy propagation is obtained when theone-half length, wave length spacing of the outer charges 17 from thecenter charge 17 is used and the charges are simultaneously detonated.In accordance with still another feature of the present invention,suppression of undesired energy propagation laterally from the shotholes is also obtained by employing the detonating or firing systemillustrated in Fig. 4 of the drawings to detonate the center charge 17bin the center shot hole 15a at a different time than the outer charges17:; in the shot holes 16a. In brief, the firing system shown in Fig. 4of the drawings comprises a current source 26 for energizing theignition elements of the charges 17a and 17b, an on-oif switch 27 and acommutating device for energizing the ignition element of the centercharge 17]) either before, after or at thesame time as the outer charges17a are energized. This commutating device is adapted to be driven aboutits axis of rotation 31 by means of any conventional constant speedmotor and is comprised of a slip ring 28 having a commutating segment28a interposed between insulating rings 29 and 30. The slip ring 28projects radially beyond the ends of the rings 29 and 30 and theprojecting end portion is constantly engaged at its inner periphery by acarbon brush 32. 'Two additional brushes 33 and 34 are provided whichare spring urged to engage the periphery of the insulating ring 30 andto be successively engaged by the commutating segment 28a of the ring 28during each revolution of the commutator assembly. With thisconstruction and with the commutating device 25 in operation, circuitsare successively completed for energizing the ignition element of theouter charges 17a and then the ignition element of the center charge 17bin response to closure of the switch 27. Thus, by observation of thecommutating device (which travels at observable speed in-acounterclockwise direction), the switch 27 may be closed while thecommutating segment 23 is traveling awayfrom the brush 34 and toward thebrush 33. With this switch closed, a circuit including the brush 32, thecommutating ring 28, the segmentZSa and the brush 33, is first completedfor energizing the ignition elements of the four outer charges 17a inseries, thereby to effect simultaneous detonation of these charges, andsimultaneously to transmit a time break signal over the circuit leads25a to the distant amplifying and filtering apparatus. A predeterminedtime interval thereafter (this interval is determined by the speed ofthe commutating device 25 and the spacing between the brushes 33 and34), the commutating segment 28a engages the brush 34 to energize theignition elements of the charge. 17b in series with the currentlimitingresistor 35 (which is equal in resistance to the combined resistance ofthree of the ignition elements) to efiectdetonation of the charge. Thedelay interval separating detonation of the center and outer charges is,of course, determined by the spacing between the brushes 33 and 34andthe speed of the commutating device, and for any given speed may bechanged as desired by varying the spacing between the brushes 33 and 34.Preferably, therefore, the brush 34 is arranged for adjustment aroundthe ring 30 to any desired position on either side of the brush 33 or tothe same position as the latter brush, so that by appropriate adjustmentof. the brush 3d, the center charge 17b may be detonated any desireddetermined time interval before or after detonation of the outerchargef17a or simultaneously with these charges. a l

'Detonating the center and outer charges in sequence,,. with varyingdegrees of delay between the detonation of center charge 17b and thesimultaneous detonation of the outer charges 17a has the effect ofchanging the'character of the directionalization of the explosive energyin a downward direction. Specifically and as best shown in Fig. 5 of thedrawings, when any given charge is detonated, a wave front is generatedwhich travels symmetrically outward in all directions away from the shotpoint at the same speed. In reference to the charge 17b, this wave frontin travelling away from the charge is represented by a series ofexpanding circles 36. Two similar series of expanding circles 37 and 38represent travel of the wave fronts away from the left and right shotpoints, respectively, when the charges 17a at these points aredetonated.

It is the positions of the meeting points of the Wave fronts producedthrough detonation of the center and outer charges relative to thelocations of these charges which determine the degree ofdirectionalization of the propagated energy. Thus with the center charge17b disposed in the earths crust at alesser depth than the outer charges17a, the wave fronts developed upon simultaneous detonation of thecenter and outer charges will meet at points between the center andouter charges and equidistant therefrom. In other words, the Wave frontsof the center and outer charges meet at some point along each of thelines 39a and 3% which are the locus lines of the points of intersectionof the wave front circles 36 individual to the center charge 17b withthe wave front circles 37 and 38 of the same diameter individual to theleft and right outer charges. The angle of convergence between the lines39a and 3% thus may beused as an index of the degree ofdirectionalization of the generated explosive energy in a downwarddirection.

Considering the circle diagram shown in Fig. 5 further, it will beunderstood that if detonation of the center charge 17b is delayed for aninterval after the outer charges 17a are fired, the wave frontstravelingaway from the outer shot points 17a will be further removed from theirrespective source points than the wave front traveling away from thecenter shot point 17b at the meeting points between the center and outerwave fronts. Thus for one delay interval, the wave fronts generated upondetonation of the outer charges 17a will meet the wave front generatedupon detonation of the center charge 17b at corresponding points alongthe locus lines 43a and 43b. If a longer delay interval is used, Wavefront collision will occur at corresponding points along the locus lines41a and 41b. 0n the other hand, if the center charge 17b is detonatedahead of the outer charge 17a, the wave front traveling away from thecenter shot point will be further removed from this point than the wavefronts traveling away from the respective outer shot points at thepoints of collision therebetween. Depending upon the magnitude of thisdelay interval, wave front meetings may occur at corresponding pointsalong corresponding locus lines 4211-421), 43a43b, etc.

From the above explanation with reference to Fig. 5

' of the drawings it will be clearly apparent that the degree ofdirectionalization of explosive energy propagation in the desireddownward direction may be. changed as desired by changing the timerelationship between the instant of detonation of the center charge'17band the instant of detonation of the outer charges 17a. In general, bestresults are obtained when the center and outer charges are detonated toproduce wave front meeting at the same points along loci lines having anangle of from 15 to 60 convergence. Another factor or parameter whichdetermines the degree of directionalization of explosive energypropagation is the relative depth of the center and outer charges. Thiswill be clearly evident from a consideration of Fig. 6 of the drawingswherein the center charge 17b is shown as being at the same depth as theouter charge 17a. From a comparison of the lines 39:: and 39brepresenting the loci of the wave front .rneeting' upon. simultaneousdetonation'of the center and outerl'charges"having the"same depth asshown in'Fig.v

decreased; The same effect is observed from a coniparison of "thecorresponding locus lines of Figs/ and 6 re spectively, representingdelayed center charge detonation and delayed outer charge detonation. Itwill thus be apparent that the relative depths of the center and outercharges is a second factor or parameter which may be varied to governthe degree of explosive energy directionalization which may be obtainedupon detonation of the several charges. At appreciable distances awa Ifrom the shot holes, this factor has no observable effect upon thecancellation of undesired waves in a horizontal direction, i. e.,laterally of the shot holes.

As indicated above, however, the time relationship between the firing ofthe center charge and the several outer charges has a marked effect uponthe suppression of undesired energy propagation in a horizontaldirection away from the shot holes for'the reason that this relationshipdetermines the phase relationship between waves radiating from theseveral shot points following the initial impulse or transient wave.Thus if the waves radiating from the center shot hole are in phase withthe'waves radiating from the outer shot holes 16a, undesired energy isadditive. On the other hand and as explained above, if the wavesradiating from the center hole 15a are out. of phase with thoseradiating from outer shot holes 16a, the waves tend to cancel and energysuppression is obtained. This latter eitect may be realized in themanner explained above by making the radius of the circle on which theouter shot holes 16a are located equal to one half the wave length ofthe waves representing the undesired energy. The desired out of phaserelationship may also be obtained by properly delaying detonation of thecenter charge 17b for a predetermined interval after the outer charges1712 are detonated. in general, maximum undesired energy propagation ina horizontal direction is obtained when the following relationshipprevails:

-a-zr DT- V where DT=the delay interval in milliseconds separatingdetonation of the outer charges 17a and later detonation of the centercharge .17 b; R-==radius ofthe. circle on which the outer shotholes 16aare. located; V =velocity' of wave propagation in the horizontaldirection in feet per millisecond; aswave length of the undesired waveenergy.

From this relationship, it will be observed that any desired radius R ofthe circle on which the outer shot holes 16a are located may be used,providing an appropriate adjustment is made in the factor DT to producesuppression of undesired energy propagation in a horizontal direction.

To summarize, two variable factors or parameters, i. e., the factor DTand R govern undesired energy suppression. Further, the two factors DTand the relative depths of the center and outer explosive charges may bevaried to obtain the desired degree of directionalization of explosiveenergy propagation in a downward direction. Permissible variations inthese three parameters permit optimum resultsin the form of maximum sup:pression of undesired waves and maximum directionalizamacaw tion oftheexplosive energyfin a downward direction to be obtained, Thus,-as'sume'-that"a vertical 'co-p'lanar explosive charge arrangement of'theform shown in Fig. 4 of the'dra'wingsis employed using only threecharges.

"Withthe wave length'of the undesired energy known from test dataobtained in 'the manner explained above, the outer shot holes may bespaced apart a distance equal to one half' the wave length of theundesired wave energy. When this spacing is used, maximum cancellationof undesired wave energy is obtained in the manner previously explainedwhen these two outer charges are simultaneouslydetonated. Withthe'center explosive charge 17b located midway between the two outercharges 17a, the delay' time or factor DT' necessaryto produce undesiredwave cancellationas between the center charge 17b and each of the outercharges 17a may be readily determined from the above equation and thebrushes 33 and 34 of the commutatingidevice correspondingly adjusted toproduce this delay interval, 'At this point, the necessary stepshavebeen taken'to produce maximum suppression of undesired wave energypropagationin a horizontal direction. There remains. only the step ofobtaining maximum.directionalization in a downward direction of V theexplosive energy developed upon detonation of the trated in Fig. 3 ofthe'drawings.

charges. This is accomplished by appropriate selection of the onlyremaining unfixed parameteni. e., the depth of the center charge 17brelative to the depth of the two outer charges. 17a. This parameter maybe determined on an experimental basis to produce maximumdirectionalization of the; explosive energy downward.

Although the desired ends may be accomplished in the manner describedabove through the use of nonshaped explosive charges, they arematerially enhanced by employing cavitated charges of the characterillus As there shown, the explosive charge 17 is comprised of a body ofexplosive material-18, such, for example as Seismogel A, manufactured byE. I. du Pont de Nemours and Company, Inc., or the like, disposed withina closed container 19 and adapted to be detonated by means of aconventional cap 20 embedded in the top portion thereof. This capincludes the usual electrical ignition element adapted to beelectrically connected to the detonator 14 by means of a pair ofinsulated conductors forming a cable 14b. At the bottomend thereof, thebody of explosive material 18 is' provided with a conical cavity 21which may be formed through the use of a metallic conical shaped linerhaving its peripheral edge joined to the walls of the container 19 at apoint removed from the lower end of this container. 'In order to enhancethe directional explosive effect obtained through provision of theconical cavity in the lower end of the explosive body 18, the mouth ofthis cavity should be separated from the adjacent. bottom .of the .shothole by a predetermined standolf distance. This is convenientlyaccomplished by extending' the. sides of the container 19'and bysecuring stand-olfprongs 22 .to the bottom wall of the container toproject downwardly therefrom. These prongs may be forced into the bottomof the shot hole in order fixedly to position the charge 17 with itscavity 21 directed downwardly. In general, the stand-elf distance willdepend upon the diameter of the explosive body 18 and upon the angle ofthe conical concavity 21 in the lower end of this body. Experienceindicates that for a 60 cone angle of the cavity 21, the optimumstand-01f distance should be approximately one and one-half times thediameter of the body 18. As will be understood, if the enclosedexplosive charges are to be used in shot holes containing looseor fluidmaterial, at least the stand-off portion of the container should besealed. In addition, if the closed stand-off portion of the container issealed and evacuated, a greatly increased directional efi'ect isobtained. The type of explosive used is not critical although best-results have been obtained-with those explo sives which have thehighest velocity of detonation and are reasonably plastic. The lastmentioned characteristic is of importance, since the contact of theexplosive with the concavity liner, if used, must be uniform over theentire area of contact between the explosive material and the liner inorder to obtain best results.

As will be understood from the above explanation, when the shot holearrangement illustrated in Fig. 4 is used, cavitated charges 17 of thespecific form just described are disposed in the shot holes 15 and 16with the prongs 22 of each charge extending into the bottom surface ofthe shot hole to insure maintenance of the charge in an upright ortilted position. After the charge is thus properly placed in the shothole, the cable conductors 14b extending to the top of the shot hole arewired to the detonator in the manner illustrated in Fig. 4 of thedrawings. When each charge is detonated, the explosive energy issubstantially all directed in a downward direction due to the effect ofthe concavity provided at the lower end of the charge. Hence arelatively small amount of seismic energy of the interfering type isgenerated and propagated laterally from the shot hole. Moreover, due tothe high degree of directionalization of the generated energy in thedesired direction, only a small amount of explosive material is requiredto produce a large amount of seismic energy propagation in a downwarddirection. In fact, in certain areas where conditions of seismic wavepropagation are highly favorable, the use of a single cavitated charge17 of the character shown in Fig. 3 of the drawing and utilizing only arelatively small amount of explosive material may be found to produceentirely acceptable seismic records without resorting to use of themulti-shot systems illustrated in Figs. 1, 2 and 4 of the drawings.Depending upon conditions, the single cavitated charge 17 as used insuch areas may be disposed in shot holes drilled through or onlypartially through the weathering layer 9. In any case, however, a markedsaving in the amount of explosive material required is realized.

From the foregoing explanation it will be understood that the presentinvention represents a marked improvement in the art of seismic wavegeneration in that it permits a much more efiicientuse of explosiveswith an accompanying decrease in the amount of interfering energyproduced. Also, material labor savings may be realized by virtue of thefact that shallower shot holes may be used without affecting thecharacter of the seismic records obtained.

While different embodiments of the invention have been described, itwill be understood that various modifications may be made thereon whichare within the true spirit and scope of the invention as defined in theappended claims.

We claim:

1. A system for producing directional propagation of seismic wavesthrough the earth, comprising a pair of spaced apart explosive chargesdisposed in the earths crust at approximately the same depth, a thirdexplosive charge disposed in the earths crust at a depth not greaterthan the depth of said pair of charges and substantially equally spacedfrom said pair of charges, and means for simultaneously detonating saidpair of charges and for detonating said third charge a predeterminedtime interval thereafter, thereby to directionalize the major portion ofthe explosive energy in a downward direction.

2. A system for producing directional propagation of seismic wavesthrough the earth and for at least partially suppressing horizontalpropagation of certain undesired waves of a predetermined frequency,comprising a pair of explosive charges disposed at approximately thesame depth in the earths crust and spaced apart a distance equal tosubstantially one-half the wave length of waves of said predeterminedfrequency, whereby waves of said predetermined frequency horizontallypropagated from said charges upon simultaneous detonation thereof tendto cancel, a third charge laterally displaced from said pair 10 ofcharges in the earths crust at a point substantially equidistant fromsaid pair of charges, and means for simultaneously detonating said pairof charges and for detonating said third charge a predetermined timeinterval thereafter to effect at least partial cancellation of the wavesof said predetermined frequency horizontally propagated from said thirdcharge by the waves of the same frequency horizontally propagated fromeach of said outer charges, the depth of said third charge being sorelated to the depths of said pair of charges that the major portion ofthe explosive energy developed upon detonation of said charges isdirectionalized in a downward direction when said third charge isdetonated said predetermined interval after said pair of charges.

3. The method of generating seismic waves in the earth, which comprisespositioning three charges substantially in alignment and in spacedrelationship, detonating the two outer charges simultaneously, therebyto propagate explosive forces from the outer charge location pointstoward a common meeting zone, and detonating the center charge apredetermined time interval after detonation of said outer charges topropagate explosive forces from the center charge location point to saidcommon meeting zone which interact with the forces arriving from saidouter charges to produce resultant forces directionalized away from saidcommon meeting zone and said center charges.

4. The method of generating seismic waves in the earth, which comprisespositioning two explosive charges in spaced relationship a distanceequal to an integer multiple of one half the Wave length of undesiredwaves produced upon detonation of said charges, thereby to effect atleast partial out-of-phase cancellation of said undesired waves,locating a third explosive charge between said charges in a position todirectionalize into the earth the forces resulting from interaction ofthe explosive forces developed upon simultaneous detonation of said pairof charges and later detonation of said third charge, simultaneouslydetonating said pair of charges, and detonating said third charge apredetermined time interval after detonation of said pair of charges toeffect at least partial out-of-phase cancellation of undesired waveenergy developed upon detonation of said third charge with undesiredwave energy developed upon detonation of said pair of charges.

5. The method of generating seismic waves in the earth and of at leastpartially suppressing horizontal propagation of certain undesired wavesof a predetermined frequency, which comprises drilling a group of shotholes around the circumference of and at the center of a circle having aradius equal to substantially one half the wave length of the waves ofsaid predetermined frequency, the depths of said holes beingsubstantially equal, depositing explosive charges in said holes,simultaneously detonating the charges in the outer holes, and thereafterdetonating the charge in the center hole, thereby to effectdirectionalization of the explosive energy produced by said charges in adownward direction.

6. The method of generating seismic waves in the earth and of at leastpartially suppressing horizontal propagation of certain undesired wavesof a predetermined frequency, which comprises drilling a group of shotholes at spaced points around the periphery of a surface of revolutionand at a point along the axis of said surface of revolution, saidsurface of revolution having a radius equal to substantially one halfthe wave length of the waves of said predetermined frequency at thevelocity of propagation in the strata beneath the weathering layer ofthe earth, said holes all extending through the weathering layer and thedepths of said holes being substantially equal, depositing explosivecharges in said holes, simultaneously detonating the charges in theouter holes, and thereafter detonating the charge in the centerhole,thereby to effect directionalization of the explosive energy produced bysaid charges in a downward direction.

7. 'Lhenrethod of generating seismic waves in the earth and of at leastpartially suppressing horizontal propaga tion of certain undesiredwavesof a predetermined frequency, which comprises drilling a group ofshot holes around the circumference ofand at the center of a circlehaving a radius equal to substantially one half the wave length of thewaves of said predetermined frequency, the depths of said holes beingsubstantially equal, depositing explosive cavitied explosive charges insaid holes with the cavitied sides thereof facing downward,simultaneously detonating the charges in the outer holes, and thereafterdetonating the charge in the center hole, thereby to efiect ,directionalization of ,the explosive energy developed'by ehar'ges'in adownward direction.

References Cited in the file of this patent UNITED STATES PATENTSMcCollum Mar. 7;

Stoneking Apr. 22,

Farnham Feb. 1,

Hawkins et al Mar. 3,

FOREIGN PATENTS Germany Mar. 24,

